Many tanks for storing liquids, particularly in the oil and gas industry, are associated with combustible vapors. Such tanks include, but are not limited to, oil production tanks, produced water and condensate tanks and the like. Hydrocarbon vapors collect in the head space of the tank. The tanks might be vented to atmosphere, but more preferably, for environmental and possibly regulatory reasons, the vapors need to be eliminated, such as through combustion in a flare.
Such tanks are usually operated at low pressures, such as just over atmospheric, the pressure therein numbering in mere inches of water column. Optional pumps or blowers can be used to raise scavenged vapors to higher pressure, such as to about 15 psig, for delivery and destruction in a vapor recovery unit such as an incinerator. Combustion of low vapor gases is typically a continuous process, in burners configured for low pressure fuel and air.
Further, it is known to blow down high pressure apparatus and flare the gases, either prior to equipment repair or for safety release of gas, such as in emergency situations. Apparatus which may require blow down include high pressure vessels, including but not limited to high pressure separator vessels, reboilers and the like. Blow down may also be required for high pressure gas transport lines. Such flaring is used to avoid unburned discharge of the high pressure vapors into the atmosphere. Typically, high pressure gas flaring is conducted using a simple open pipe discharge and igniter system, the gas stream being occasional, of short duration yet at high flow rates. Flaring produces significant radiant heat and are therefore elevated above the ground to space the source of heat from ground-based equipment and personnel.
Low pressure hydrocarbon gases having a molecular weight ratio to carbon of less than about 0.3 tend to produce smoke as a result of incomplete combustion and the formation of free carbon. Thus, where low pressure gases are to be combusted, either a smoke suppressant, such as steam or water, is known to be added or air is introduced to provide turbulent mixing with the hydrocarbon for minimizing smoke during combustion. High pressure hydrocarbon gases however are generally different as, upon release, there is sufficient kinetic energy to minimize smoke.
It is known to use separate flare combustors to handle each of low pressure and high pressure vapors however there is a high initial capital cost and ongoing maintenance costs associated with supporting separate stacks.
Others have attempted to provide single combustors which combust both low pressure vapors and high pressure vapors in the same stack. One such combustor is taught in Canadian Patent CA 1085709 to John Zink Company USA wherein a dual pressure burner is mounted at a top of a stack, the low and high pressure gases being supplied using separate lines extending through the stack to intermingled nozzles for simultaneous combustion at the same burner or burners. Air is aspirated by induction through the inside of the stack to low pressure gas discharge apertures. In another embodiment, a single relief line is used however this embodiment can only be used when the low and high pressure are not burned simultaneously. A blower is used to provide air to minimize smoke.
As taught in U.S. Pat. No. 3,822,985 to Straitz, low pressure gas is supplied through a conduit which extends within and to the top of the stack and high pressure gas is supplied through a separate conduit which extends along an outside and to the top of the stack, both low and high pressure gases being delivered for combustion at the top of the stack. A burner assembly at the top receives high pressure gas for combustion. Further low pressure gas is supplied through vanes, both low and high pressure gases being burned using the same burner assembly at the top of the stack.
Further, as taught in U.S. Pat. No. 3,822,984 to Straitz, the high pressure gas can be fed through a supply line extending inside the flare stack to a series of vanes at the top of the flare stack which provide a swirling action to the high pressure gas. The low pressure gas is fed through a separate supply line also extending inside the flare stack to a ring at a top of the stack. A series of continuously burning pilot burners ignite the low pressure gas. If high pressure gas is to be combusted it is supplied outwardly through the vanes and is ignited with the low pressure gas. If the flame begins to smoke, steam is provided through a plurality of steam nozzles surrounding the pilot burners.
Applicant believes that there is in risk in co-discharge of low and high pressure gases as pressure differential or backflow along the low pressure can trigger release valves in low pressure equipment resulting in a release. Further, such release valves, after exposure to the nature of high gas pressures, can remain stuck open even when the pressure returns to the normal low pressure, resulting in a continuing release of gas to the atmosphere.
Further, combustion efficiency is a concern. Current regulations for flaring of gas, whether it is low pressure or high pressure gas, require a minimum of 98% efficiency in Canada and a minimum of 95% efficiency in the USA. Along with being smokeless, the flame should not be visible at a top of the stack when combusting the low pressure gases. During emergency high pressure flaring situations, which are typically short in duration, flame is permitted to be visible above the top of the stack and the flame may produce smoke. It is desirable however that the amount of smoke be reduced or eliminated.
Clearly, there is interest in less costly, single flare apparatus which is capable of combusting both low and high pressure gas and which is able to meet or exceed the current stringent environmental and regulatory requirements and reduce or eliminate smoke production.